WORCESTER — A judge has issued a ruling precluding prosecutors from using a police officer's observations of blood as evidence against two of the three men charged with murder in the 1994 disappearance of Kevin Harkins.

Matteo Trotto, John Fredette and Elias Samia are awaiting trial in Worcester Superior Court on first-degree murder charges in what prosecutors allege was the Feb. 15, 1994, shooting death of the 36-year-old Mr. Harkins, whose body has never been found.

The prosecution contends Mr. Harkins, who was officially declared dead in 2008 in Worcester Probate and Family Court, was fatally shot three days after reneging on a promise to offer false testimony in a drug case that was then pending against Mr. Fredette.

Prosecutors allege Mr. Harkins was shot in the back of the head while in Mr. Samia's car with Mr. Samia, Mr. Trotto and Mr. Fredette after having been called out of Suney's Pub on Chandler Street by Mr. Trotto. They further charge that Mr. Samia disposed of the car and canceled its registration the next day, after the vehicle had been pulled over by a Millbury police officer who said he saw blood in the 1985 Chevy Impala and on Mr. Fredette's leather coat.

Lawyers for the three suspects sought to suppress all evidence emanating from the traffic stop, including former Officer Mark S. Moore's stated observations of blood and statements attributed to Mr. Samia and Mr. Fredette. Mr. Trotto was not in the car at the time of the stop.

The defense alleged in its motion to suppress that the stop of the vehicle and the search and interrogation that followed were illegal. Prosecutors argued that the actions of police were justified and asked the court to deny the motion.

Judge Shannon Frison, who presided over a hearing on the motion, allowed it, in part, as it related to Mr. Samia and Mr. Fredette in a Nov. 7 ruling. The judge denied the motion as it related to Mr. Trotto, finding that because he was not present and had no possessory interest in the vehicle, he lacked standing to challenge the legality of the stop and search.

The ruling would appear to clear the way for prosecutors to seek to introduce the disputed evidence at Mr. Trotto's May 15 trial. Mr. Samia and Mr. Fredette are to be tried separately from Mr. Trotto and a date for their trial has not been set.

Either side could appeal Judge Frison's ruling.

Mr. Moore testified during the hearing that he pulled the car over for speeding on Route 20 in Millbury about 2 a.m. Feb. 16, 1994. The retired officer said he became suspicious that the vehicle might have been stolen after Mr. Samia was unable to produce the registration, saying he had given it to his insurance company because he had recently had the car painted.

Sgt. Stephen Webb soon arrived at the scene to back Officer Moore up.

When asked where Mr. Samia and Mr. Fredette were coming from, Mr. Samia said the Anchor Lounge in Millbury, according to Mr. Moore. He said he found this response suspicious, as well, given the vehicle's route of travel.

Mr. Fredette was unable to produce any identification, but identified himself by name, Mr. Moore said. The former officer said he then asked Mr. Samia if he had any "guns, drugs, bombs, tanks or dead bodies" in the vehicle, a question, he said, that often elicited a laugh from a driver and made for a more relaxed discussion.

He said he asked Mr. Samia several times if he could search the car and Mr. Samia repeatedly declined to consent to a search, asking each time why the officer wanted to conduct such a search. Mr. Moore said he told Mr. Samia he thought he was lying about where he was coming from.

Still concerned that the car might have been stolen, Mr. Moore said he went around the vehicle and ordered Mr. Fredette to get out so he could safely look at the back of the inspection sticker inside the car to determine whether the vehicle identification number and plate number on the sticker matched those on the vehicle.

The plate number on the sticker did not match the plate number on the car, according to Mr. Moore.

While in the car, Mr. Moore said he detected an odor of marijuana and determined he had probable cause to search the vehicle. He said he found a cannister containing marijuana residue in the car and saw blood on the front passenger seat and floor and on the driver's side door panel.

Blood was also seen on the sleeve and back of Mr. Fredette's leather coat when Sgt. Webb searched him after he got out of the vehicle, according to Mr. Moore.

When asked about the blood, both men said they had gotten into a fight about two hours earlier at the Harding Café in Worcester and Mr. Fredette had been punched in the nose, Mr. Moore testified. The search of the vehicle did not include the trunk or glove box because both were locked and Mr. Samia said he did not have a key for either, the former officer said.

Mr. Moore said Mr. Fredette had no visible signs of injury to his face and that the amount of blood on his back was not consistent with a nose bleed.

He said he and Sgt. Webb eventually determined they did not have probable cause to make an arrest and Mr. Samia and Mr. Fredette were allowed to leave.

Mr. Moore said he did not cite Mr. Samia for speeding or not having his registration in the vehicle. He said he confiscated the marijuana residue and later destroyed it at the police station. He testified that he did not write a report about the stop until about a month later, when he was asked to do so by Worcester Police Sgt. Timothy O'Connor, since retired, who was investigating Mr. Harkins' disappearance.

Citing case law, Judge Frison said Officer Moore's actions were legally justified up to the point where he ordered Mr. Fredette from the car and got in to look at the back of the inspection sticker.

"In light of these cases, it is clear that Officer Moore overstepped the permissible bounds of inquiry by ordering Mr. Fredette out of the car and entering it himself to check the information on the back of the inspection sticker. Accordingly, Officer Moore's actions constituted an unreasonable search," the judge wrote.

She ruled that all evidence acquired "after the unlawful intrusion," including Mr. Moore's stated observations of blood, "must be suppressed as a fruit of the poisonous tree."

Timothy J. Connolly, a spokesman for District Attorney Joseph D. Early Jr., said today the ruling is under review and no decision has been made on a possible appeal.

Mr. Trotto's lawyer, Joseph A. Franco, could not be reached for commments.